In order to feed and fuel a growing world population against a backdrop of threats to harvests from climate change requires a dramatic leap in crop production of 40-50% by 2030 (IAASTD report (2009) Agriculture at a crossroads). Limitations of nutrient, light and water availability can have drastic detrimental effects on crop yield. In North America alone it is estimated that 40% of annual crop losses are due to low water availability (IAASTD report (2009) Agriculture at a crossroads; Lynch (2011). Plant Physiol 156:1041-9; Lynch (2013). Annals of Botany doi:10.1093/aob/mcs293).
Similarly, sub-optimal levels of the essential nutrients phosphate and nitrogen require the application of fertilisers that are expensive to produce, have a significant carbon footprint, and can adversely affect natural habitats through run-off from farmland. The urgent need to improve crop performance while seeking to reduce fertiliser and irrigation inputs has brought renewed interest in the manipulation of structural traits to maximise nutrient-use efficiency, water-use efficiency and optimise the capture of solar energy. While the number and length of lateral branches produced by the plant have an important bearing on the extent of possible occupation of above and below ground space, it is the angle of growth of those branches that is the major determinant of the efficiency and effectiveness of resource capture both above and below ground (Lynch (2011) Plant Physiol 156:1041-9; Lynch (2013). Annals of Botany doi:10.1093/aob/mcs293; Liao et al. (2001) Plant and Soil 232:69-79; Liao et al. (2004) Functional Plant Biology 31:959-70; Ouyang et al. (2011). J. Integrative Agriculture 1701-9). For example, shallow rooting genotypes in maize and bean display up to a 50% increase in yield low phosphate soils (Liao et al. (2001) Plant and Soil 232:69-79; Liao et al. (2004) Functional Plant Biology 31:959-70).
The approach of breeders and biotechnology companies to the optimisation of crop architecture via the manipulation of GSA in crops has been severely limited by the complete lack of knowledge of the mechanisms underlying the regulation of branch angle. Hence approaches have been restricted to conventional breeding practices that are time-consuming and lack the precision required to optimise growth angle without compromising other performance traits.
The most desirable growth angle habit for root and shoot varies according to the agricultural environment. For example, in dry soils it can be advantageous to have steeper, more vertical lateral roots while, as noted above, in low phosphorous soils, a shallower, less vertical lateral root system can increase phosphate uptake thereby reducing or eliminating the need for additional inputs. Importantly, there are also crop-specific ideals for branch angle. For example, in oilseed rape a more vertical branch angle in the shoot canopy is desired because it improves light penetration during the crucial pod-filling phase prior to harvest.
The overall architecture of higher plants is determined by the number and arrangement of lateral branches around the main root-shoot axis. The principal function of these shoot and root branches is to hold leaves and other organs to the sun, and below ground, to facilitate the uptake of nutrients and water, and provide secure anchorage for the plant. Most commonly, these lateral root and shoot branches are set and maintained at specific angles with respect to gravity, a quantity known as the gravitropic setpoint angle (GSA). While the GSA of the primary root and shoot is typically approximately vertical, the GSA values of lateral shoots and roots are most often non-vertical, allowing the plant to optimise the capture of resources both above- and below-ground. Despite the importance of branch angle as a fundamental parameter of plant form, until recently the mechanism underlying the setting and maintenance of non-vertical GSAs was not known. A defining characteristic of the GSA concept is that upon being displaced from its GSA, an organ will rapidly undergo a gravitropic response to return to its original angle of growth with respect to gravity (Digby R D and Firn J. (1995). Plant Cell Environ. 12:1434-40; Cline M G (1996). Ann Bot (Lond) 78: 255-266). For the primary axis, in which vertically growing roots and shoots have a GSA of 0° and 180° respectively, this is readily accounted for by the well-supported model for gravitropism proposed by Cholodny and Went (Mullen J P and Hangarter R, (2003). Advances in Space Research. 31:2229-2236; Morita M T (2010). Annu. Rev. Plant Biol. 61:705-20; Blancaflor E. B., Masson P. H. (2003). Plant Physiol. 133: 1677-1690): the orientation of shoots and roots is monitored in specialised gravity-sensing cells called statocytes within which starch-rich bodies called statoliths sediment according to the gravity vector. As such statoliths provide a biophysical sensor of statocyte orientation within the gravity field and displacement from the vertical leads to the PIN auxin efflux carrier-mediated movement of auxin to the lower side in both root and shoot tissue. This auxin redistribution generates an asymmetry in auxin-regulated gene expression between upper and lower tissues that drives organ-level bending growth (Morita M T (2010). Annu. Rev. Plant Biol. 61:705-20; Blancaflor E. B., Masson P. H. (2003). Plant Physiol. 133: 1677-1690). In the shoot, auxin promotes cell elongation, causing upward bending, while in the root auxin inhibits cell elongation, causing downward anisotropic growth (Morita M T (2010). Annu. Rev. Plant Biol. 61:705-20; Blancaflor E. B., Masson P. H. (2003). Plant Physiol. 133: 1677-1690). The magnitude of this gravitropic response can in many cases be described by sine law, formulated by von Sachs in 1882, which states that the strength of the gravitropic response is dependent on the sine of the initial displacement angle (Sachs J (1882). Arb Bot Inst Würzburg 2:226-284. While there are species-specific differences in the range of displacement angles over which the sine law applies (Sachs J (1882). Arb Bot Inst Würzburg 2:226-284; Galland P. (2002) Planta 215: 779-784), as a general principle it is the case that the greater an organ is tilted away from its GSA, the greater the magnitude of the gravitropic response (Sachs J (1882). Arb Bot Inst Würzburg 2:226-284; Galland P. (2002) Planta 215: 779-784).
In contrast to the primary axis, the robust maintenance of growth at non-vertical GSAs in lateral organs cannot be explained by standard model Cholodny-Went-based gravitropism as described above. We recently established the existence of a mechanism, the anti-gravitropic offset (AGO), that counteracts gravitropic response specifically in the gravity-sensing cells of lateral branches but not in those of the primary root-shoot axis. Our key findings can be summarised as follows:
i. Gravity-dependent non-vertical growth of lateral root and shoot branches is sustained by an anti-gravitropic offset (AGO) mechanism that operates in tension with gravitropic response to generate net non-vertical growth.
ii. The activity of the AGO requires auxin transport.
iii. The angle of growth of lateral root and shoot branches is dependent on the magnitude of this anti-gravitropic offset component; a stronger AGO induces less vertical growth and vice versa.
iv. Auxin regulates the magnitude of the AGO and hence the GSA of lateral branches, separately from driving the anti-gravitropic growth itself.
v. Auxin's regulation of the AGO is effected, via TIR1/AFB-mediated transcriptional control, specifically in the gravity sensing cells of the root and shoot.
We have elucidated the means by which the growth angle of lateral root and shoots can be made either more or less vertical. The invention is therefore based on the finding that expressing regulators of auxin signalling in the gravity-sensing cells of the shoot and the root in higher plants is sufficient to alter the angles of lateral root and shoot branches in these species. For example, this may lead to plants having more vertical or less vertical lateral root and shoot branches. The targeted manipulation of branch angle traits may play a significant role in enhancing water and nutrient acquisition and photosynthetic efficiency in plants (including crops) grown in a range of agricultural conditions.